Sapphires and rubies are forms of the mineral corundum, A1,0g. Corundum is one of the hardest known natural materials.
The hardness of fonic compounds depends on the structure of the crystal
and the strength of the ionic bonds. The distance between the aluminum
lons (AP) and oxide ions (027 is 191 pm. Use Coulomb's law to explain how
the bonding in Al2O, influences its properties.

the tides are influenced by both the moon and the sun, it's easy to see that when the sun lines up with the moon and the earth, as during a new moon or full moon, the tidal effect is increased. these are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. on the other hand, if the sun and the moon are 90 degrees apart in relation to an observer on earth as during the first quarter moon or third quarter moon, then high tides are not as high as they normally would be. this is because despite its greater distance, the sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the moon's pull. this phenomenon of lower high tides is called a neap tide. if you need more information on this topic visit moonconnection.com.